This invention relates generally to peptides and more specifically to peptides which are involved in cell adhesion systems.
The adhesion of cells to other cells or to extracellular materials is important to the orderly development and functioning of cells in an organism. Cell adhesion is mediated by certain adhesive legands and corresponding receptors. Among such adhesive ligands are the glyco-proteins fibronectin, vitronectin and collagen. All three contain the tripeptide sequence arginine-glycine-aspartic acid (Arg-Gly-Asp or R-G-D) which appears to function as the primary recognition site for receptors on the surface of cells binding to these molecules. When presented to cells as a peptide-covered surface, synthetic peptides containing the Arg-Gly-Asp sequence promote cell attachment in a manner similar to that of natural fibronectin or vitronectin. In solution, such peptides can inhibit cell attachment to a surface coated with fibronectin, vitronectin, collagen, the peptides themselves or some other adhesive protein having an Arg-Gly-Asp cell attachment site.
Several receptors have now been identified which recognize the Arg-Gly-Asp sequence of their respective ligands. While some of these receptors have affinity for only their specific ligand, others will interact to varying degrees with two or more ligands containing the tripeptide. It therefore appears that while the Arg-Gly-Asp sequence is sufficient for receptor recognition and binding, the remaining amino acid sequence of the peptide may nonetheless be important to the specificity of the ligand-receptor interaction. The precise manner in which the remaining sequence affects binding has, thus far, remained elusive.
One view that has been held by some investigators in the field is that the specificity of a given adhesive protein for its receptor may depend on the presence in the adhesive protein of one or more receptor binding sites other than their Arg-Gly-Asp site. According to this view, the Arg-Gly-Asp site would be a shared binding site and the additional site or sites would be responsible for the specificity (See, for example, Yamada et al., (1987) Biochem. and Biophys. Res. Comm. 144:35; Wright et al., (1987) PNAS 84:1965). An alternative possibility is that the Arg-Gly-Asp sequence provides essentially all of the information for the receptor binding and that it is the conformation of this sequence that gives an adhesion protein its receptor specificity. The binding sites of various peptide hormones are known to be small (Rose et al., (1985) Adv. in Chemistry 37:1) but these peptides generally comprise only about 10 amino acids in total. In contrast, each of the two fibronectin polypeptide chains comprises over 2000 amino acids. The idea that the conformation of a single amino acid triplet could be important to the function of a number of different proteins carrying it is novel.
Synthetic peptides containing the Arg-Gly-Asp sequence are useful in that they can both promote and inhibit cell attachment. There are at least ten different adhesion receptors that are known or suspected to recognize an Arg-Gly-Asp sequence in one or more of the various adhesive proteins. Peptides that reproduce with reasonable accuracy the function of such adhesive proteins or specifically inhibit that function offer a tremendous potential for the manipulation of important physiological events such as thrombosis, metastasis, inflammation, wound healing, and rejection of prosthetic implants.
There thus exists a need for peptides having an amino acid structure that provides the optimum specificity for the receptor of interest. The present invention satisfies this need and provides related advantages as well.
The present invention involves novel synthetic peptides which have high affinity and specificity for their receptors by virtue of restrictions on their stereochemical conformation. Such restrictions, or stabilizations, can be provided, for example, by cyclization, by inclusion into a constraining conformational structure such as a helix, by providing an additional chemical structure such as an amide or an enantiomer of a naturally occurring amino acids, or by other methods. Such peptides can have a range of numbers of residues, but preferably between 3 and 100, more preferably 7 to 30. In particular, there is provided a cyclic peptide having increased affinity and selectivity for the vitronectin receptor over that of linear, Arg-Gly-Asp-containing synthetic peptides. The affinity and selectivity of the cyclic peptide approaches that of natural vitronectin.
In one aspect, the invention comprises a peptide having about 10 amino acids in which a bridge is formed between amino acids surrounding the Arg-Gly-Asp sequence. A suitable structure is 
The peptide may have about 1 to 4 amino acids between the Arg-Gly-Asp sequence and those residues forming the bridge. The bridging residues may be, for example, penicillamine and cysteine, or other amino acids capable of forming a bridge, such as a disulfide bridge.
Another suitable structure is 
Alternatively, the peptide may be cyclerized through a peptide bond between amino acid residues. An appropriate structure is 
The present invention provides evidence that, rather than the specificity of adhesion ligands residing in distinct binding sites outside the Arg-Gly-Asp site, such specificity results from the conformational structure imposed on the Arg-Gly-Asp sequence by the structure of the remaining peptide. At the practical level, the invention demonstrates that a conformationally restricted Arg-Gly-Asp-containing peptide can have different receptor specificity and higher binding affinity than its unrestricted counterpart. This demonstration permits the synthesis of other Arg-Gly-Asp conformations with variant receptor specificities and affinities.
The stabilized peptides of the present invention have various applications relating to their cell-adhesion properties. In one embodiment, for example, where the peptide structure is 
the peptide has increased affinity for the vitronectin receptor and decreased affinity for the fibronectin receptor over that of linear synthetic Arg-Gly-Asp-containing peptides and may therefore be effectively used to inhibit the binding of vitronectin receptor-containing cells to a substrate for cell culture. Alternatively, the stabilized peptide may be usefully employed to promote adhesion of cells expressing the vitronectin receptor by, for example, coating cell culture substrates. Additionally, the stabilized peptide may be used to coat materials for in vitro implantation, such as prothesis, where cell attachment is desired.